Combination power amplifier



Dee. A27, 1949 EB. STONE 2,492,542

COMBINATION POWER AMPLIFIER ATTORN EY Dec. 27, 1949 F. B. STONE COMBINATION POWER AMPLIFIER JFiled' sept. '7, 1945 3 Nw Nw mw E E E -uw R l Sv bb un Patented Dec. 27, 1949 COIWBINATION POWER AMPLIFIER Fred B. Stone, Haddon Heights, N. J., assignor to Radio Corporation of Amcricaa corporation of Delaware Application September 7, 1945, Serial No. 614,955

Claims. (Cl. 179-171) My present invention relates .generally to irnprovements in audio frequency amplifier systems, and more particularly to improvements in volume adjusting devices for audio frequency amplifier systems.

It is an important object of my present invention to provide an audio amplifier system having relatively high audio output capability with negligible harmonic distortion over the range of low volume adjustment.

Another object of my invention is to provide an audio frequency amplifier system between a source of the audio frequency signals and a reproducer for such signals, the vamplifier system being characterized by the fact that is consists of a pair of channels of adjustable amplitude transmission, such channels having substantially different characteristics relating harmonic distortion to output power.

o In accordance. with still another object of my invention, a pair of separate audio frequency signal amplifier channels areconnected in parallel, one of the channels having appreciably less harmonic distortion at low signal output amplitudes than the other channel, and there being utilized respective, uni-controlled volume control devices of such construction that negligible signal input exists at the input terminals of the amplifier channel having the higher harmonic distortion until` the output of .the klowerl distortion channel contains harmonic distortion comparable to that of the higherY distortion channel at relatively low output levels. f

Other objects, advantages and features will appear in the following description, taken in con-- nection with the drawings, in which I have indicated diagrammatically a circuit organizationwhereby my invention may be carried into effect.

Referring-to the drawings:

Fig. 1 shows schematically an audio frequencyamplifier system embodying my invention;

Fig. 2 depicts harmonic distortion vs. output power characteristics for the parallel signal amplifier channels shown in Fig. l;

Fig. 3 shows the manner of constructing .the volume control devices; 'l

Fig. 4 illustrates the resistance vs.- angle of rotation characteristics for the respective Volume control devices shown in Figure 3;

Fig. 5 shows idealized characteristics of Fig. 4, each volume control resistor beingassumed to consistvof additional resistance sections; and

Fig. 6 illustrates the relationships between the angular settings of the tandem volume control and the respective outputs, including that at harmonic frequencies as well as the combined output, from both amplifiers.

In Fig. 1 I have shown the various networks of the system embodying my invention, the networks being schematically represented. The schematic represenation is believed suflicient to indicate to those skilled in the art of constructing and operating an audio frequency amplifier system the nature of the circuits used in the various networks indicated. The numeral I denotes a source of audio frequency signal currents. The source I may be of any suitable type. For example, it may be the audio demodulator or detector of a radio receiver, whether of the amplitude modulated type or frequency modulated type. The source I may, also, be the electrical pickup device of a disc or film type of sound reproducing system, or it may be the microphone of a public address system or industrial sound system. My invention is independent of the specific construction or electrical connections of the source of audio frequency currents. Indeed, the source I may even extend beyond the audio frequency range, and hence the expression signal currents is to be understood as comprising a range of frequency components extending into the superaudible frequency band.

The numeral 2 denotes a reproducer for translating the signals of source I into sound waves, where the signals at source I are in the audio frequency range. The reproducer 2 may be of any suitable type. In the case of a radio receiver, it will be the usual receiver loudspeaker. Between the transducers I and 2 there is connected the amplifying system embodying my present invention. Essentially the amplifier system consists of a pair of amplifier networks having respective volume control, or signal output level adjusting, devices. The amplifier networks are connected in parallel relation, and the characteristics thereof are correlated in a predetermined manner with the respective volume control devices thereof.

-The amplifier network A is essentially characterized by the fact that it has a low produc-- tion of harmonic distortion at relatively low signal output levels. It is provided with a volume control device which consists of a resistor Ri having a slider or adjustable contact 4. The resistor R2 is connected by resistance R and lead 5 to the high potential output terminal of source I, while the lower end of resistor R2 is grounded. In other words the device R2, 4 is a potentiometer, or potential divider, device shunted across source I. The adjustable tap 4 is connected by lead 6 is a conventional, relatively low-cost amplifierv such as might be designed for radiov receivers or public address systems in which due to cost factors involved, the distortion 'at'low signal levels cannot be limited to a negligilo'ly low value and' the ampliiier still be built to provide high power output. Amplifier A on the other hand Vdoes notV have to supply a high output (for example, about 5 to 8 Watts maximum lfor radio receiver' use), and, therefore, low distortion can be given greater consideration in the construction ofv amplifier A.

The amplier E is provided with its respective potentiometer, or potentialdi'vi'der, whose un'- grounded end of resistor R1 is yconnected in conimon with lead 5 to the ungrounded terminal ofso'urce l. The slider 9 of potentiometer resistor R1 is connected by lead lll to the ungro'unded input terminal of amplifier B. The output termi-b nalsof ampliner B connected in paralielwit-n the output terminals of amplifier A to the input terminals of reproducer 2. The numeral I l desige nates schematically, by' a dashed line, any suitable mechanical adjusting device for concure rently varying the positions of Sliders 4 arid 9 on their respective resistors Rz and Ri. p

In Fig. e I nave indicated the respective volume control resistors R1 and R2 with their correspending sliders s and A. It will be seen that the common 'adjusting means il concurrently 'varies the angular rotation of sliders 9 and '4. The angle of rotation of each slider with respect to the glided Ed Of its reSp'eCtil/rleslstafi has been indicated by the letter 01 In the present exempllii-cation of my invention, resistor Ri has been shown as consisting', in order between lead 5 and ground, of a medium resistance section l2, a high resistance section I3v and a low resistance section i4. As the slider 9 is moved upward vin position from the grounded end ne d first increases, and then over the upper part of the range decreases to Van intermediate 'vali-ie.

The resistor Ri has a high resistance section i3' at its ground end, a low resistance section le at its ungrouncled end, and a `i'iiediui'n 1resistance section l2" between the other sections. In this case decreases as 0 increases. A resistor R in series with Ri functions to limit the input to the low distortion amplifier A so that the latter may not be overloaded, at least until the higher distortion ampiiner B is badly overloaded also. For this reason resistor R is connected between lead and the upper end of resistance section M'. It

is pointed out that overload of the low distortion ampliiier A can be prevented by limiting the input thereto as shown in Fig. 1, or by proportioning the gain of the amplifier channel A with respect to channel B such that both amplifier channels preierably overload at about the same point.

In Fig. 2 I have depicted typical characteristics for amp-liners A and B. The characteristic curves in Fig. 2 for the amplier'networks relate Watts output against Percent total harmonic distortion. It will be noted that amplifier channel .Al-has a slowly rising distortion characteristic for low output levels so that for low signal output Vthe'harriioriic distortion is negligibly low. On the 'other hand, amplifier channel B has a relatively high harmonic distortion percentage output for relatively low signal. output amplitudes, while at higher output levels the harmonic distortion is proportionately higher.

From this graph chart it is seen that over the lower-outputran'ge (say up to 5 watts) ampliiier A shows' considerably ies's. distortion than would be obtained freinY amplifier' B worked over the Same Output range.

Ampliiier B may be of any suitable constructien, as, for example, the type of audio frequency amplifier commonly employed "in a radio receiver. Such an ampli'er usually employs pentodes, or beam power tubesvin theV output stage. Y In such an amplier the' distortion although low may lie appreciable evenV at `low outputV levels, and may ycontain nigh orderA components. Amplifier A is constructed lso as tohave very low amplitude distortion especially at the low signal output levels, and this-may be done, for example, by employing a properly designed triode class A amplifieri system.- An amplifier employing two type 45 trioelesv -in'pusnpull maybe used. Reference is :made to RCA applicationnote No. 40 entitled "Application note'on nigh power output from type 4-5 tubes (Copyrightige by RCA Radictron Co. Inell, particularly to Fig. 2C thereof, for suitable characteristics to enable one skilled ln the art to design a suitable low distortion amplifier. in

" that typeY of amplier' circuit the power output is V5 wat-ts with 0.6 percent distortion. Such a system will give less objectionable distortion at low vamplitudes than for channel B.

The class A system becomes costly, however,

whenitrequired to deliver high power due to the lower plate enlciency. Alternatively, amplifier B maybe a so=called class B amplier of the type employing a suitable tube, e. g., the 46 type triode using zero grid bias. "Such a class B amplifier has 'a relatively high total percent harmonic distortion at power outputs up to 2- watts. In generati, do not wish to limit my invention to any particular tubes or circuit connections in amplifier networks A `and B', since those skilled in the artof radio `communieation will readily be able to construct suitable amplifier networks from Ythe characteristics orFig. 2' and 'my aforesaid description.

The two signal voutput amplitude control cevices vare so constructed that substantially no signal inputis applied to 'amplifier B until ampliner A is operating at a fairly high level of signal output. Preferably, signal is delivered to the amplifier B at the point where 'the harmonic distortion produced 'at mplier A has. attained the value of distortion which amplifier B exhibits at thelower'end o'ffits signal output range. This e'rectcanbe accomplished :by constructing resistors't-i yand Ri fas shortn.V As previously explained, the rotation of sliders 9 and il do not produce like resistance changes.V Theresistors'are provided with such relative tapering that the aforesaid relation of signal input to amplifiers A and B is secured. Y,

In Fig. 4 thereis shown apair of'curves relatins Angle of rotation andv Percentage of total resistance ofeach volume control Ri and Rz, the latter term referring to the resistance between the slider arms and ground in relation to the total resistance. will be noted that the volume control device for amplifier B has substantially less change in resistance than the volume control device for amplier A forAthe early part of the angle of rotation of adjusting device II. At 270 of rotation both volume control devices have maximum resistance magnitude. The respective use of three resistive sections I2, I3 and I4 provides the characteristics shown in Fig. 4. By the use of a number of additional resistive sections for the two voltage dividers R1 and Re smoother control characteristics can be obtained approaching those shown in Fig.r5.

In Fig. 6 I have chartedthe relationships between the angularsettings of the tandemvolume control and the respective outputs, including that at harmonic freouencies vas well as .the combined output, from both ampliiiers. The abscissae are Angle of rotation,'while.the two ordinates are R. M. S. power output-watts-fundamental plus harmonics and R. M. S. power output at harmonic frequencies-watts.l AThe calculated relationships are based on full output from both ampliers when the volume control is at the maximum volume setting. At lower volume settings, the power output will vary directly as the square of the percentage values of Fig. 4.

The amplifier B does notstartto contribute. as can be seen from the lcurves ofFig. 6, until the tandem volume control has been rotated through about 105 degrees of the vavailable 270 degrees rotation. AAt that point the output from amplier A is 3.5 watts with milliwatts harmonic output (about 0.6 percent distortion). At 170 degrees rotation'both amplifiers develop about the same power' (6.5 watts), and have approx'imately the sameharmonic output. Over the remainder of the volume control range of 1'70 to 270 degrees, the output from amplifier B predominates.

Note that the volume control for amplifier B is constructed to decrease the slope of the output versus angular rotation characteristics over the last 70 degrees rotation (Fig. 4). This shape of control characteristic is necessary in order to have the output of ampliiier B coordinate in the desired manner with that of amplifier A, and use the full 270 degrees rotation. This shape of characteristic is, also, desirable from the standpoint of smooth overload.

By means of the present arrangement it is possible to secure relatively undistorted signal input at the input terminals of the reproducer 2 at normal output levels with an amplifier of limited power capability. From the curves of Fig. 2 it is seen that at high output signal levels, where very low harmonic distortion is not required, the signal output is supplemented by amplier B. At the low signal output levels of the parallel channels the signal input to amplifier B is negligible, while the amplifier A which receives the signal input produces negligibly small harmonic distortion.

While I have indicated and described a system for carrying my invention into eifect, it will From these curves it 6, be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention.

What I claim is:

1. In an amplifier system oi the type employing a source of electrical signals including frequency components of a predetermined frequency range and a reproducer for such signals; the improvement comprising at least two amplifying channels, each of said channels being provided with a pair of input terminals and a pair of output terminals, means lfor connecting the output terminals of said channels in parallel connection to said reproducer, one channel being constructed to produce relatively low harmonic distortion at low signal output levels, a second channel being of relatively high power output and of such construction that the distortion at 10W signal levels cannot be limited to a value as low as that of said one channel, a first volume control device coupling said source to the input terminals of said one channel, a second volume control device coupling said source to the input terminals of the second channel, a common means connected to adjust said volume control devices for selectable overall power output levels, and the volume control device electrically associated with the second channel be- Aing so constructed and arranged that substantially no signals are applied to the second channel at low volume adjustment of said common means while signals are delivered vthereto at the point where the harmonic distortion produced in said one channel has attained the value ofvdistortion which the second channel exhibits at the lower end of its signal output range.

' 2. In an amplier system ofthe type employing a source of electrical signals including frequency components of a predetermined frequency range and a reproducer for such signals; the improvement comprising at least two amplifying channels, each of said channels being provided with a pair of input terminals and a pair of output terminals, means for connecting the output terminals of said channels in parallel connection to said reproducer, one channel being constructed to produce relatively low harmonic distortion at low signal output levels and having limited power output, a second channel being of relatively high power output but the distortion at low signal levels being substantial, a rst volume control device coupling said source to the input terminals of said one channel, a second volume control device coupling said source to the input terminals of the second channel, common means connected to simultaneously adjust said volume control devices for selectable overall power levels, the volume control device electrically associated with the second channel being constructed and arranged to provide substantially no signal input to its associated channel at low volume adjustment of said common means and substantial signal input when the output of the first channel contains harmonic distortion comparable to that of the second channel at relatively low output levels, each of said volume control devices including a fixed resistor shunted across said source and a respective slider, and the electrical tapering of said xed resistors being opposite over at least a portion of the provided range of adjustment.

3. In a system of the type including a source of audio frequency currents and an audio frequency reproducen, a, rst audio signal trans-j mission channel. including a high output levelamplifier whose distortion: at low signal output levels cannot be` limited to a relatively low value, a signal amplitude'adjiisting means coupling said source to the input terminals of said amplifier, said amplitude adjusting means being constructed toy provide relatively negligible signal input to the amplifier at low amplitude settings thereof, a second signal transmission channel including an amplifier of relatively low harmonic distortion at low signal output levels, means feeding; the signal output-of said two channels to said reproducer, a second signal amplitude adjusting device coupling the input terminals of said source to said second. channel amplifier, the second amplitude .adjusting device being constructed to provide substantial signal input to said second channel amplifier at low amplitude settings thereof, andmeans for conjointly varying theA settinge of said two signal amplitude adjusting devices.

4. In an amplifier of the type employing a source: of electrical signals including frequency components of a predetermined frequency range and a reproducer for such signals; an improvement comprising at leastftvvo` amplifying channels including aseparate output stage for each channel, means for connecting the output terminals of. vsaid. channels in parallel to said reproducer, one of saidchannels being designed so as to producerelatively low harmonic distortion especially at: low output amplitudes, the other-channel being designedl for relatively high power output but with higher harmonic distortion resulting from the requirements for such power output7 means for coiuiectirig the inputs of the two channels to said source o f signals, said last means consisting of separate volume control devices, the volume control devices being so constructed and relatively proportioned that substantially no input signals. areapplied tothe higher distortion ampliiler until the output signals from the lower distortion amplifier attain a predetermined amplitudevalue andas thev signals fed from the source to the two channels are further increased in amplitude the rate of. increase to the higher distortionamplier increases over at least ay portion of the provided .range yoadjustment While the rate of increase. to, the low distortion amplifier decreases, for preventing' overloadingv of one channel substantially before the other.v

5. In` a high-level ampliilcation system havingv a controllable power Youtput level for amplifying electric signals and delivering the amplified signals at a selectable intensity level; a 10W-distortion amplification channel in which harmonic distortion is minimized and the resulting power output rapidly becomes excessively distorted when raised to relatively limited power output levels; aV high-power output amplification channel in which the output4 intensity is maximized and the resulting harmonic distortion is relatively high even atv lowr output levels;` the high-power channel having a maximum power output higher than the low-distortion channel, said channels having input and voutput terminals connected for amplifying signals yin parallel; output level control elements Vfor each channel; and selector means having a single settable member for operating the control elements to furnish the desirecl, overall power output level, said selector means being connectedV for utilizing essentially the low-distortion channel output at low overall power level settings', for adding' high-power channel output when theV settings increase to an overall power level Aat `which the distortion ofthe lowdistortion channel' reaches; approximately the distortion level of the high-power channel, and at still higher settings to addv high-power channel output andlow-dlst'ortion channel. output in relative amounts that prevent excessive total distortion.

FRED B. STONE.

REFERENCES CITED The following references are of recordV in the file ofthis. patent:

UNITED STATES PATENTS Wolff oet. 27, 1942 

